In recent years, piston noise complaints have been on the rise. Piston noise includes “piston slap” and wrist pin knock or rattle. These noises are most frequently generated upon cold starting of the engine, but can also be manifest on hot restarts. Objection to piston noise continues to be a source of customer complaints. Even though normal piston noise is not indicative of eminent mechanical failure, customers may deem it as unacceptable and the engine as lacking quality.
Modern piston noise can be attributed to lateral instability of the piston assembly and lack of sufficient lubrication within the critical interfaces of the piston-to-bore and wrist pin joints. Severe packaging constraints and ever increasing power demands have led to very short piston designs with rotund skirt profiles. Furthermore, a challenging high temperature environment now exists for the piston and pin, which requires some means for supplemental cooling. In the past, the automotive piston/pin assembly has relied primarily on “splash” lubrication for cooling and noise control. However, relatively dry cylinder bores and pin joints have resulted for a number of reasons. Most notable of these contributors are tight crank bearing clearances (for low crank system noise) and aggressive piston ring designs (for reduced oil consumption). Unfortunately, these necessary refinements exacerbate the dry scenario for the reciprocating hardware. This is especially the case upon engine start up, as immediate lubrication is critical for “cushioning” the relevant interfaces involved with piston noise.
The advent of polymer coated piston skirts has enabled much tighter piston fit tolerances, which has addressed the aspect of piston stability with a remarkable reduction in piston noise. However, pin noise remains and piston noises can still be of concern in certain instances. Therefore, additional lubrication has become a fundamental requirement for the contemporary high performance engine. The most viable means of supplying added lube to the reciprocating hardware include rifle drilled rods, connecting rod squirters, and full time block-mounted piston oilers.
Rifle drilled rods are less frequently employed in automotive engines than block squirters. This design includes a passage drilled through the entire length of the rod's column, thus connecting the wrist pin end to the big end of the rod. Oil is fed up through the center of the rod and directed as necessary to facilitate pin lubrication and/or to cool the piston under-dome. This technology is often used in large HD diesel engines. Its main advantage is communicating lubricant directly and internally right to the point of use for maximum effectiveness. The largest deterrents to gun drilled rods is the cost associated with drilling such a long, small diameter passage. The scrap rate can be excessive in weight conscious designs.
The most popular means for supplying added lube to the reciprocating hardware is connecting rod squirters, which incorporate a small orifice along the side of the rod. Rod squirters emit an intermittent spirt of oil, once per engine revolution, whenever the squirter hole in the rod aligns with the drilled lube passage in the crank's rod journal. Properly timed and targeted, the rod squirter can provide ample lube for the piston squirt thrust surfaces as well as for the wrist pin joints. The main advantages of rod squirters are that they usually package better than block squirters and do not place a huge demand on the oil supply system (i.e., the oil pump). Additionally, rod squirters are generally less expensive than block squirters.
Full time block squirters consist of a nozzle that is mounted in the crankcase, near the bottom of each cylinder, which directs a steady stream of oil to the bottom side of the piston dome. To alleviate excessive demands on the oil pump, usually the nozzle head incorporates a check ball valve assembly. These check valves typically begin to flow when the supply pressure exceeds around 25 psi (175 kPa). The main benefit of block squirters is that of piston cooling, which can lower critical piston surface temperatures by 30° C. Disadvantages of common block squirters are that their targeting is much less effective for cold noise control, and they are difficult to package. Quite often, block squirters mandate that a notch be provided at the lower end of the piston squirt for clearance at bottom dead center. This is undesirable as it creates a stress riser in an area of the piston skirt, which is already under high stress. Further, block squirters are typically more expensive to implement and somewhat more likely to malfunction due to a plugged or sticky check valve.
All contemporary block-mounted squirters orient the discharge from the nozzle straight up the center of the bore such that the oil stream impinges on the underside of the piston dome for maximum cooling. The sprayed lubricant is in continuous contact with the underside of the dome for the entire stroke of the piston. This type of spray provides little, if any, lubrication to the piston skirt-to-bore interface. With the piston in the vicinity of top dead center, oil must be present within this interface immediately upon a cold start-up of the engine to minimize noise. With block-mounted squirters targeted straight up the center of the piston, the oil is disbursed but virtually all of it is contained within the piston's cavity. Essentially none of the oil is splashed onto the bore walls. This problem is exacerbated upon cold starts and under low engine speed conditions wherein traditionally targeted block squirters do not distribute oil high enough up on the cylinder walls nor when it is most needed for piston noise control.